U.S. patent number 10,323,294 [Application Number 15/753,307] was granted by the patent office on 2019-06-18 for austenitic stainless steel foil.
This patent grant is currently assigned to NIPPON STEEL & SUMIKIN MATERIALS CO., LTD.. The grantee listed for this patent is NIPPON STEEL & SUMIKIN MATERIALS CO., LTD.. Invention is credited to Naoki Fujimoto, Masahiro Fukuda, Toru Inaguma, Naoya Sawaki, Hiroto Unno, Tomohiro Uno.
United States Patent |
10,323,294 |
Unno , et al. |
June 18, 2019 |
Austenitic stainless steel foil
Abstract
Provided is an austenitic stainless steel foil that demonstrates
a high degree of stretch formability and little deformation
anisotropy with respect to stretch forming despite having a sheet
thickness of 60 .mu.m or less. The austenitic stainless steel foil
of the present invention has a sheet thickness of 5 .mu.m to 60
.mu.m, a recrystallization rate of 90% to 100%, and a texture in
which the total of the area ratio of a crystal orientation in which
the difference in orientation from the {112}<111> orientation
is within 10.degree., the area ratio of a crystal orientation in
which the difference in orientation from the {110}<112>
orientation is within 10.degree., and the area ratio of a crystal
orientation in which the difference in orientation from the
{110}<001> orientation is within 10.degree., in a measuring
field thereof, is 20% or less.
Inventors: |
Unno; Hiroto (Tokyo,
JP), Sawaki; Naoya (Tokyo, JP), Fujimoto;
Naoki (Tokyo, JP), Fukuda; Masahiro (Tokyo,
JP), Uno; Tomohiro (Tokyo, JP), Inaguma;
Toru (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMIKIN MATERIALS CO., LTD. |
Tokyo |
N/A |
JP |
|
|
Assignee: |
NIPPON STEEL & SUMIKIN
MATERIALS CO., LTD. (Tokyo, JP)
|
Family
ID: |
58052183 |
Appl.
No.: |
15/753,307 |
Filed: |
August 17, 2016 |
PCT
Filed: |
August 17, 2016 |
PCT No.: |
PCT/JP2016/074027 |
371(c)(1),(2),(4) Date: |
February 17, 2018 |
PCT
Pub. No.: |
WO2017/030149 |
PCT
Pub. Date: |
February 23, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180237882 A1 |
Aug 23, 2018 |
|
Foreign Application Priority Data
|
|
|
|
|
Aug 17, 2015 [JP] |
|
|
2015-160687 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D
8/0205 (20130101); C21D 8/0268 (20130101); C21D
6/008 (20130101); C21D 8/0236 (20130101); C21D
8/0273 (20130101); H01M 2/0285 (20130101); C21D
8/0405 (20130101); H01M 2/02 (20130101); B21B
1/38 (20130101); B21B 1/40 (20130101); B21B
1/22 (20130101); C21D 1/74 (20130101); C22C
38/44 (20130101); C22C 38/001 (20130101); C22C
38/04 (20130101); H01M 10/052 (20130101); C21D
8/0447 (20130101); C22C 38/58 (20130101); C21D
6/005 (20130101); C22C 38/02 (20130101); C21D
6/004 (20130101); C21D 8/0468 (20130101); B32B
15/08 (20130101); C22C 38/42 (20130101); C21D
1/76 (20130101); C21D 8/0436 (20130101); C21D
9/48 (20130101); C22C 38/00 (20130101); B32B
15/085 (20130101); C21D 9/46 (20130101); C22C
38/40 (20130101); H01M 2/0202 (20130101); C22C
38/002 (20130101); B32B 15/18 (20130101); C21D
2201/05 (20130101); Y10T 428/26 (20150115); Y10T
428/264 (20150115); Y10T 428/12951 (20150115); Y10T
428/12569 (20150115); Y10T 428/12993 (20150115); Y02E
60/10 (20130101); Y10T 428/265 (20150115); Y10T
428/24967 (20150115); B32B 2457/10 (20130101); Y10T
428/2495 (20150115); C21D 2211/001 (20130101); Y10T
428/12972 (20150115) |
Current International
Class: |
C22C
38/44 (20060101); C22C 38/42 (20060101); C22C
38/04 (20060101); C22C 38/02 (20060101); C22C
38/00 (20060101); H01M 2/02 (20060101); B32B
15/085 (20060101); C21D 9/48 (20060101); C21D
8/02 (20060101); C21D 1/74 (20060101); B32B
15/08 (20060101); B32B 15/18 (20060101); C21D
9/46 (20060101); C22C 38/58 (20060101); B21B
1/22 (20060101); B21B 1/38 (20060101); B21B
1/40 (20060101); C21D 8/04 (20060101); C22C
38/40 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2157195 |
|
Feb 2010 |
|
EP |
|
2000-273586 |
|
Oct 2000 |
|
JP |
|
2003-239044 |
|
Aug 2003 |
|
JP |
|
20015-342693 |
|
Dec 2003 |
|
JP |
|
2004-52100 |
|
Feb 2004 |
|
JP |
|
2007-168184 |
|
Jul 2007 |
|
JP |
|
2010-194759 |
|
Sep 2010 |
|
JP |
|
4608256 |
|
Jan 2011 |
|
JP |
|
2012-92361 |
|
May 2012 |
|
JP |
|
2013-41788 |
|
Feb 2013 |
|
JP |
|
WO 2015/122523 |
|
Aug 2015 |
|
WO |
|
Other References
Extended European Search Report, dated Jan. 28, 2019, for
corresponding European Application No. 16837138.3. cited by
applicant.
|
Primary Examiner: La Villa; Michael E.
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Claims
The invention claimed is:
1. An austenitic stainless steel foil having a sheet thickness of 5
.mu.m to 60 .mu.m, comprising: a recrystallization rate of 90% to
100%, and an texture in which a total of an area ratio of a crystal
orientation in which a difference in orientation from a
{112}<111> orientation is within 10.degree., an area ratio of
a crystal orientation in which a difference in orientation from a
{110}<112> orientation is within 10.degree., and an area
ratio of a crystal orientation in which a difference in orientation
from a {110}<001> orientation is within 10.degree., in a
measuring field of the stainless steel foil, is 20% or less.
2. The austenitic stainless steel foil according to claim 1,
wherein a resin film is laminated on at least one surface of the
austenitic stainless steel foil.
3. The austenitic stainless steel foil according to claim 1,
wherein the sheet thickness is 5 .mu.m to 25 .mu.m.
4. The austenitic stainless steel foil according to claim 3,
wherein the number of crystal grains in the direction of sheet
thickness is 3 or more, and wherein the number of crystal grains in
a direction of sheet thickness is determined by measuring crystal
grain size in compliance with JIS G 0551, calculating average
crystal grain size, dividing sheet thickness by average crystal
grain size, and using the quotient thereof as the number of crystal
grains in the direction of sheet thickness.
5. The austenitic stainless steel foil according to claim 3,
wherein a nitrogen concentration of a surface layer of the
stainless steel foil is 1.0% by mass or less.
6. The austenitic stainless steel foil according to claim 3,
wherein a resin film is laminated on at least one surface of the
austenitic stainless steel foil.
7. The austenitic stainless steel foil according to claim 1,
wherein the number of crystal grains in the direction of sheet
thickness is 3 or more, and wherein the number of crystal grains in
a direction of sheet thickness is determined by measuring crystal
grain size in compliance with JIS G 0551, calculating average
crystal grain size, dividing sheet thickness by average crystal
grain size, and using the quotient thereof as the number of crystal
grains in the direction of sheet thickness.
8. The austenitic stainless steel foil according to claim 7,
wherein a nitrogen concentration of a surface layer of the
stainless steel foil is 1.0% by mass or less.
9. The austenitic stainless steel foil according to claim 7,
wherein a resin film is laminated on at least one surface of the
austenitic stainless steel foil.
10. The austenitic stainless steel foil according to claim 1,
wherein a nitrogen concentration of a surface layer of the
stainless steel foil is 1.0% by mass or less.
11. The austenitic stainless steel foil according to claim 10,
wherein a resin film is laminated on at least one surface of the
austenitic stainless steel foil.
Description
TECHNICAL FIELD
The present invention relates to an austenitic stainless steel
foil. More particularly, the present invention relates to an
austenitic stainless steel foil provided with favorable formability
despite having an extremely thin sheet thickness.
BACKGROUND ART
Batteries such as lithium ion batteries installed in numerous
electronic devices are required to have compact size and light
weight in response to the increasingly high levels of portability
and mobility of these devices accompanying reductions in size and
weight. The reductions in battery size and weight required by
electronic devices such as smartphones in particular require
leading-edge specifications.
The battery cases of lithium ion batteries currently available for
use in smartphones use a can-shaped aluminum thin sheet or aluminum
foil laminated with a resin film. Resin film-laminated aluminum
foil is frequently used for the purpose of improving capacity
density per volume in particular. More recently, even thinner
cladding materials are being required for the purpose of further
reducing size and weight. However, when the thickness of the
aluminum foil serving as the base material is reduced, problems
occur such as increased susceptibility to the occurrence of
pinholes during the course of production, being unable to ensure
moisture impermeability, reductions in puncture strength and
rigidity, and being unable to ensure adequate strength with respect
to external impacts and internal expansion of the battery.
Consequently, aluminum foil is considered to have reached its limit
with respect to further reductions in thickness.
Therefore, attention has been focused on the use of foil made of
stainless steel (stainless steel foil) since it demonstrates higher
rigidity and strength than aluminum. However, since stainless steel
has higher specific gravity than aluminum, in order to apply
stainless steel to battery cases required for use in increasingly
compact and lightweight electronic devices, stainless steel foil is
required that has an extremely thin sheet thickness (such as 60
.mu.m or less). In order to increase battery capacity in
particular, stainless steel foil is required that it should enable
high processability in terms of, for example, demonstrating uniform
formability when formed into the shape of a square can despite
having a sheet thickness of 60 .mu.m or less.
Stainless steel foil having a thickness of 25 .mu.m or less is
disclosed in Patent Document 1 as an example of ultrathin stainless
steel foil. When the thickness of stainless steel is made to be
extremely thin, voids form accompanying the formation of cracks
extending from etched edges in the direction of rolling. The
invention disclosed in Patent Document 1 solves this problem by
limiting the number of inclusions that measure 5 .mu.m or larger in
size.
In addition, examples of applying stainless steel foil to a battery
case are disclosed in Patent Documents 2 to 4. Patent Document 2
discloses an example of a battery outer jacket member formed by
pressing stainless steel foil having a thickness of 20 .mu.m to 100
.mu.m, Patent Document 3 discloses that an example of a battery
jacket material formed by pressing stainless steel foil having a
thickness of 100 .mu.m, and Patent Document 4 discloses that an
example of a battery jacket material formed by pressing stainless
steel foil having a thickness of 40 .mu.m to 150 .mu.m.
PRIOR ART DOCUMENTS
Patent Documents
[Patent Document 1] Japanese Unexamined Patent Publication No.
2000-273586 [Patent Document 2] Japanese Unexamined Patent
Publication No. 2004-52100 [Patent Document 3] Japanese Unexamined
Patent Publication No. 2013-41788 [Patent Document 4] Japanese
Unexamined Patent Publication No. 2012-92361 [Patent Document 5]
Japanese Unexamined Patent Publication No. 2007-168184
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
Ultrathin stainless steel foil is usually subjected to punching or
etching directly after rolling without being post-annealed or after
being subjected to heat treatment for improving tensile strength
and proof stress in the manner of tension annealing, in many cases,
such as the springs used for the head suspension of hard disk
drives (HDD). The art disclosed in Patent Document 1 solves these
technical problems that occur during etching.
However, in the case of applying ultrathin stainless steel foil to
a battery case, the battery case is formed by press-forming the
ultrathin stainless steel foil. Press forming is typically
classified into deep drawing and stretch forming. Deep drawing
comprises deforming the material by subjecting to tensile
deformation in the inflow direction of the material and subjecting
to compressive deformation in the widthwise direction perpendicular
to the inflow direction as is typically represented by cupping deep
drawing. On the other hand, stretch forming comprises forming in
which the surface perpendicular to the direction of sheet thickness
of the foil (to also be referred to as rolled surface) is subjected
to equibiaxial tensile deformation. Since stretch forming is
carried out in the case of forming into the shape of a square can
in the manner of a battery case, the portions of the stainless
steel foil located at the corners in particular are subjected to
the greatest amount of tensile deformation. Consequently, if a
large number of crystal grains oriented in a direction unfavorable
for deformation are present at those portions, problems such as
fracturing end up occurring since the stainless steel foil is
unable to deform sufficiently even if subjected to press forming.
Thus, it is preferable that the stainless steel foil to be
subjected to processing in which the rolled surface is stretched in
an arbitrary direction has an excellent stretch formability in an
arbitrary direction rather than the stretch formability in a
specific direction, or in other words, has an excellent stretch
formability with little anisotropy.
As a result of conducting extensive studies in view of the
aforementioned circumstances, the inventors of the present
invention found that, when conventional annealing treatment (such
as bright annealing) is carried out on stainless steel foil with
taking no account of treatment conditions, the crystal grains of
which the stainless steel foil is composed coarsen, and further the
orientation of these crystal grains ends up accumulating in
specific crystal orientations. As this accumulation of the
orientation of crystal grains in specific crystal orientations
progresses, deformation anisotropy with respect to stretch forming
ends up increasing. Therefore, it was considered that uniform
forming of the stainless steel sheet became difficult and forming
depth was reduced.
Patent Document 2 describes an example of pressing stainless steel
foil having a thickness of 20 .mu.m to 100 .mu.m to form into a
battery case. However, Patent Document 2 does not indicate any
awareness of the problem concerning accumulation of crystal
orientations. Consequently, annealing temperature becomes high, and
accumulation of crystal orientations proceeds. Therefore, it is
considered that the stainless steel foil of Patent Document 2
exhibits large deformation anisotropy with respect to stretch
forming.
Patent Document 3 also describes an application example of
stainless steel foil having a thickness of 100 .mu.m to a battery
case. However, since stainless steel foil having a thickness of 100
.mu.m is comparatively thick, although it demonstrates a high
degree of processability, the effect of increasing battery capacity
due to reducing case thickness is small.
Patent Document 4 describes an example of applying stainless steel
foil having a thickness of 40 .mu.m to 150 .mu.m to a battery outer
jacket member. The technology described in Patent Document 4 is
used to reduce the generation of deformation-induced martensite
during press forming by nitriding the surface layer of the
stainless steel foil. It is explained that, as a result thereof,
peeling resistance at those locations where the stainless steel
foil and resin are joined by thermal fusion bonding can be ensured
and blushing of the resin during press forming can be prevented.
Moreover, Patent Document 4 states that press formability is
improved since smoothness of the surface is maintained by
preventing from surface irregularities formed by
deformation-induced martensite transformation.
However, as a result of conducting extensive studies, the inventors
of the present invention found that, when the surface layer of
stainless steel foil is nitrided the nitrided portions become
hardened and susceptible to cause tearing (cracking) during press
forming. When the sheet thickness of the stainless steel foil
reaches 60 .mu.m or less in particular, the effect of the hardened
portions of the nitrided surface layer becomes relatively large and
no longer negligible. Namely, when ultrathin stainless steel foil
having a nitrided surface layer is subjected to press forming,
cracks are formed in the surface, and it is not able to obtain the
adequate press formability. Thus, although the thickness of
stainless steel foil can be made to be thin, it is considered that
forming depth becomes small. Namely, there is little effect of
increasing battery capacity.
Furthermore, in Patent Document 4, since nearly all sheet
thicknesses in the examples are 100 .mu.m, they cannot be expected
to demonstrate the effect of increasing battery capacity by
reducing thickness. In addition, Patent Document 4 specifies that
although formability of the example in which sheet thickness is 40
.mu.m is poor this is within an acceptable range. Moreover, since
there are no examples of thinner sheet thicknesses, the technology
described in Patent Document 4 is unable to realize both
significantly reducing the thickness of stainless steel foil and
increasing forming depth.
With the foregoing in view, an object of the present invention is
to provide an ultrathin austenitic stainless steel foil having a
sheet thickness of 60 .mu.m or less that exhibits excellent stretch
formability with little deformation anisotropy with respect to
stretch forming.
Furthermore, although there are no particular lower limit of sheet
thickness, since the realistic limit of sheet thickness of foil
after undergoing rolling is about 5 .mu.m, the thickness of the
austenitic stainless steel foil according to the present invention
is 5 .mu.m to 60 .mu.m.
Means for Solving the Problems
The inventors of the present invention obtained the following
findings by conducting extensive studies to solve the
aforementioned problems.
(a) Deformation anisotropy with respect to stretch forming can be
reduced and forming depth can be increased by making the proportion
of crystal grains oriented (accumulated) in specific directions in
austenitic stainless steel to be within a specific range, or in
other words, by making the orientation of the crystal grains to be
random while reducing dislocation density in austenitic stainless
steel.
(b) In order to allow the orientations of crystal grains to be
random while reducing dislocation density, it is necessary to
introduce numerous dislocations serving as nucleation sites during
recrystallization into the austenitic stainless steel by carrying
out rolling under high pressure, and to maintain the recrystallized
crystal grains in a fine state while recrystallizing by
subsequently carrying out annealing to reduce dislocation
density.
(c) Plastic deformability (such as favorable stretch formability)
is preferably ensured by keeping the number of crystal grains in
the direction of sheet thickness 3 or more. In addition, the lower
limit of the number of crystal grains in the direction of sheet
thickness may also be determined corresponding to sheet
thickness.
(d) It is important to minimize nitriding of the surface layer in
order to prevent from tearing (cracking) caused by surface
hardening.
(e) Resistance to electrolytic solutions can also be ensured by
keeping the number of crystal grains in the direction of sheet
thickness 3 or more and making the nitrogen concentration of the
surface layer to be 1.0% by mass or less. In other words, in order
to improve resistance to electrolytic solutions, it is important to
maintain adhesiveness with a resin coating by preventing from
roughening of the surface of the stainless steel foil at the corner
portions thereof following press forming.
The present invention was completed based on the aforementioned
findings and aspects of the present invention are as indicated
below.
(1) An austenitic stainless steel foil having a sheet thickness of
5 .mu.m to 60 .mu.m, comprising:
a recrystallization rate of 90% to 100%, and
an texture in which the total of the area ratio of a crystal
orientation in which the difference in orientation from the
{112}<111> orientation is within 10.degree., the area ratio
of a crystal orientation in which the difference in orientation
from the {110}<112> orientation is within 10.degree., and the
area ratio of a crystal orientation in which the difference in
orientation from the {110}<001> orientation is within
10.degree., in a measuring field of the stainless steel foil, is
20% or less.
(2) The austenitic stainless steel foil described in (1), wherein
the sheet thickness is 5 .mu.m to 25 .mu.m.
(3) The austenitic stainless steel foil described in (1) or (2),
wherein the number of crystal grains in the direction of sheet
thickness is 3 or more.
(4) The austenitic stainless steel foil described in any of (1) to
(3), wherein the nitrogen concentration of the surface layer is
1.0% by mass or less.
(5) The austenitic stainless steel foil described in any of (1) to
(4), wherein a resin film is laminated on at least one surface of
the austenitic stainless steel foil.
Effects of the Invention
According to the present invention, an austenitic stainless steel
foil can be provided that demonstrates an excellent stretch
formability and little deformation anisotropy with respect to
stretch forming despite having a sheet thickness of 60 .mu.m or
less. The austenitic stainless steel foil according to the present
invention is preferable for a battery case of a lithium ion battery
and the like, which is directed to reduction in size and
weight.
BEST MODE FOR CARRYING OUT THE INVENTION
The following provides a detailed explanation of the present
invention.
(1. Austenitic Stainless Steel Foil)
[Stainless Steel Material]
Although there are no particular limitations on the austenitic
stainless steel foil according to the present invention provided it
is composed of austenitic stainless steel, the austenitic stainless
steel preferably has the composition range indicated below. This
composition range comprises, by mass percent, C at 0.080% or less,
Si at 2.0% or less, Mn at 2.0% or less, P at 0.045% or less, S at
0.030% or less, Ni at 5.0% to 11.0%, Cr at 15.0% to 20.0%, Mo at
0.30% or less, N at 0.05% or less and Cu at 0.50% to 2.50%, with
the balance consisting of Fe and unavoidable impurities.
Commercially available austenitic stainless steel may also be used
for the aforementioned austenitic stainless steel.
[Sheet Thickness: 5 .mu.m to 60 .mu.m]
The sheet thickness of the austenitic stainless steel foil
according to the present invention is 5 .mu.m to 60 .mu.m. Sheet
thickness is defined to be 60 .mu.m or less in order to maximize
the effect of increasing battery capacity in the case of applying
the stainless steel foil to a battery case. Sheet thickness is
preferably 50 .mu.m or less, more preferably 40 .mu.m or less and
even more preferably 25 .mu.m or less. In addition, although there
are no particular limitations on the lower limit of sheet
thickness, in consideration of limitations on production
technology, a sheet thickness of 5 .mu.m may be set for the lower
limit. The effects of the present invention can still be enjoyed
even in the case of a sheet thickness of 5 .mu.m.
[Recrystallization Rate (Ratio): 90% to 100%]
The austenitic stainless steel foil according to the present
invention is required to have favorable stretch formability
(plastic deformability). The dislocations and other lattice defects
accumulate in the microstructure of the austenitic stainless steel
by rolling the austenitic stainless steel. Therefore, crystal
grains have high dislocation density and are hardened despite being
fine. Consequently, it is necessary to recrystallize the structure
and reduce dislocation density by properly controlling heat
treatment conditions corresponding to the material. Namely, since
the formation of recrystallized structure is driven by dislocation
density, favorable stretch formability (plastic deformability) is
ensured by preventing from the recrystallized structure from
coarsening while reducing dislocation density within recrystallized
grains.
Furthermore, although the etch-pit method is an example of a method
used to measure dislocation density, the quantitative measurement
which is carried out by this method is difficult since it is
affected by measurement conditions and other factors. Although
dislocation density can be measured directly by microscopic
observation, variations are considerable due to differences between
fields of observation. Therefore, the inventors of the present
invention found that, by measuring recrystallization rate as a
characteristic value reflecting dislocation density, it is possible
to determine whether or not proper heat treatment has been carried
out.
Recrystallization rate is calculated by dividing the area of
recrystallized crystal by the observation area. The area of
recrystallized crystal can be obtained by observing an arbitrary
cross-section of the austenitic stainless steel foil using an
optical microscope. Alternatively, it may also be calculated by
determining the half peak width of the diffraction peak of the
(220) plane obtained by X-ray analysis of the stainless steel foil.
A half peak width maximum of 0.20 degrees or less can be considered
to correspond to a recrystallization rate of 90% or more, that of
0.15 degrees or less can be considered to correspond to a
recrystallization rate of 95% or more, and that of 0.10 degrees or
less can be considered to correspond to a recrystallization rate of
100%.
The recrystallization rate of the austenitic stainless steel foil
according to the present invention is 90% or more. If the
recrystallization rate is 90% or more, dislocation density is
sufficiently low and formability can be ensured. Recrystallization
rate is preferably 95% or more. Recrystallization rate may be 100%
if the texture to be subsequently described satisfies the
requirements of the present invention. Namely, the entire
austenitic stainless steel foil according to the present invention
may be recrystallized.
(Texture)
The austenitic stainless steel foil according to the present
invention has a characteristic texture as a result of further
controlling the recrystallization process while maintaining
recrystallization rate within the aforementioned ranges. More
specifically, the austenitic stainless steel foil according to the
present invention has a texture in which the total of the area
ratio of a crystal orientation in which the difference in
orientation from the {112}<111> orientation is within
10.degree., the area ratio of a crystal orientation in which the
difference in orientation from the 11101<112> orientation is
within 10.degree., and the area ratio of a crystal orientation in
which the difference in orientation from the {110}<001>
orientation is within 10.degree., in a measuring field thereof, is
20% or less. Furthermore, in the aforementioned orientations, the
{112}, {110} and {110} planes indicate planes parallel to the
rolled surface, while the <111>, <112> and <001>
orientations indicate directions parallel to the direction of
rolling. Moreover, the aforementioned three orientations are
composed as a group of orientations that includes
crystallographically equivalent orientations.
The aforementioned {112}<111> orientation is an orientation
that is referred to as "Copper orientation", the aforementioned
{110}<112> orientation is referred to as "Brass orientation",
and the aforementioned {110}<001> orientation is referred to
as "Goss orientation". These three orientations are superior in
terms of energy and are known to be accumulated (oriented)
preferentially in the recrystallization texture of austenitic
stainless steel.
In rolled austenitic stainless steel foil, the proportion of
crystal grains oriented such that the difference in orientation
from these three orientations is within 10.degree. is low, and even
though the orientations of crystal grains in the texture are
comparatively random, formability is inferior due to high
dislocation density as previously described. Therefore, dislocation
density is lowered by promoting textural recovery and
recrystallization through annealing treatment. At this time, the
crystal grains coarsen (undergoing crystal growth) through
recrystallization. In addition, since the aforementioned three
orientations are superior in terms of energy, the proportion of
crystal grains accumulating in these three orientations
increases.
As crystal grain orientation proceeds in these specific
orientations, the crystal grains are disproportionately arranged in
a row in a specific orientation. In this case, when the stainless
steel foil is subjected to stretch forming or other press forming,
although favorable formability is demonstrated in those
orientations favorable for deformation (orientations in which
slipping occurs easily), formability becomes poor in those
orientations unfavorable for deformation (orientations in which
slipping occurs with difficulty). When the foil is subjected to
stretch forming so as to be stretched in an arbitrary direction of
the rolled surface such as the corners of a battery case, while
some orientations show adequate elongation (deformation), other
orientations unfavorable for deformation (poor plastic
deformability) may initiate fracturing and the like. Thus, a
desired forming depth may not be obtained. In other words,
deformation anisotropy occurs with respect to stretch forming.
Therefore, in order to reduce the anisotropy of formability caused
by the degree of orientation of the crystal grains, the present
invention randomizes the orientations of crystal grains in the
texture which has been recrystallized by carrying out annealing
treatment. As was previously described, since the crystal grains of
austenitic stainless steel easily accumulate in three orientations
consisting of the {112}<111> orientation, {110}<112>
orientation and {110}<001> orientation, the total proportion
of the area occupied by crystal grains considered to be oriented in
each direction (crystal grains oriented such that the difference in
orientation from each orientation is within 10.degree.) is 20% or
less. As a result, by eliminating bias in the orientation of
crystal grains in the texture of austenitic stainless steel foil
and reducing anisotropy of formability, adequate forming depth can
be obtained even in cases of being subjected to processing that
causes the rolled surface to be stretched in any arbitrary
direction. Furthermore, crystal orientations of the region which do
not accumulate such that the difference in orientation from the
aforementioned three orientations is within 10.degree. are crystal
orientations which are less easily accumulated than the
aforementioned three orientations, and crystal orientations of the
region are not accumulated in specific orientations. In other
words, in the case of an arbitrary crystal orientation X in which
the difference in orientation from the aforementioned three
orientations exceeds 10.degree., the area ratio of crystal
orientations in which the difference in orientation from X is
within 10.degree. is 20% or less.
In the present invention, the total of the area ratio occupied by a
crystal orientation in which the difference in orientation from the
{112}<111> orientation is within 10.degree., a crystal
orientation in which the difference in orientation from the
{110}<112> orientation is within 10.degree., and a crystal
orientation in which the difference in orientation from the
{110}<001> orientation is within 10.degree. in a measuring
field is preferably 15% or less and more preferably 7% or less.
In the present invention, the area ratio occupied by crystal
orientations in which the difference in orientation from each of
the aforementioned orientations is within 10.degree. can be
calculated by determining the crystal orientation at each
measurement point using electron backscatter diffraction (EBSD).
The method of measurement using EBSD measures crystal orientation
by connecting an EBSD detector to a scanning electron microscope
(SEM), inclining the sample at an angle of about 70.degree., and
analyzing the diffraction pattern (EBSD pattern) when a prescribed
measuring field on the inclined sample surface is irradiated with a
convergent electron beam.
More specifically, several orientations are estimated from the
diffraction pattern at each measurement point, and the orientation
having the highest degree of certainty is determined to be the
crystal orientation at that measurement point. The orientations at
each of the measurement points determined in this manner are then
color-coded and plotted corresponding to a location in an inverse
pole figure to obtain an inverse pole figure (IPF) map. On the
basis of this IPF map, the proportion of the area of measuring
fields occupied by crystal grains in which the difference in
orientation with each of the aforementioned orientations is equal
to or less than a tolerance angle (10.degree. in the present
invention) is calculated as the form of an area ratio. Furthermore,
in the present invention, the measuring field is preferably a
region which is about 100 .mu.m.times.100 .mu.m or larger.
[Three or More Crystal Grains in Direction of Sheet Thickness]
There are preferably three or more crystal grains in the direction
of sheet thickness in the austenitic stainless steel foil according
to the present invention. The number of crystal grains in the
direction of sheet thickness can be determined by measuring crystal
grain size in compliance with JIS G 0551, calculating average
crystal grain size, dividing sheet thickness by average crystal
grain size, and using the quotient thereof as the number of crystal
grains in the direction of sheet thickness. Furthermore, in the
case the crystal grains are equiaxed grains, average crystal grain
size may be calculated by measuring crystal grain size in the plane
perpendicular to the direction of sheet thickness.
Alternatively, three or more arbitrary lines are drawn in the
direction of sheet thickness in an arbitrary cross-section, and the
number of crystal grains traversed by these lines is counted
followed by determining the average value thereof to obtain the
number of crystal grains in the direction of sheet thickness. At
that time, crystal grains are counted as 0.5 grains in the case
they are contacting the surface. In addition, in the case a line
runs along a crystal grain boundary, each of the plurality of
crystals that compose the crystal gain boundary can also be
counted. However, since both edges of the stainless steel foil in
the widthwise direction are easily affected by annealing, they are
not suitable for measurement of the number of crystal grains.
Consequently, the number of crystal grains is preferably measured
by drawing arbitrary lines in the direction of sheet thickness
while excluding both edges of the stainless steel foil in the
widthwise direction. For example, the number of crystal grains in
the direction of sheet thickness of the stainless steel foil can be
evaluated by counting the number of crystal grains at three
locations consisting of the center in the widthwise direction of
the stainless steel foil (location located at 1/2 the width from
one edge) and locations intermediate between both edges and the
center (two locations located at 1/4 the width and 3/4 the width
from one edge), and then calculating the average value thereof.
If the number of crystal grains determined in this manner is 3 or
more, plastic deformability improves and stretch formability is
favorable. Therefore, this manner is the preferable determination.
Namely, in order for individual crystal grains to undergo plastic
deformation to an arbitrary shape, it is necessary that von Mises
conditions are satisfied and that more than one slip system causes
multiple slip. However, if there are too few crystal grains in the
direction of sheet thickness, the probability of crystal grains in
an orientation that does not satisfy von Mises conditions with
respect to the direction of deformation (crystal grains having
inferior deformability) arranging in the direction of thickness is
likely to increases. This being the case, since these crystal
grains are unable to accommodate overall deformation of the foil
during press forming, they end up becoming the origins of
fractures. On the other hand, if three or more crystal grains are
present in the direction of sheet thickness, even if crystal grains
having inferior deformability are present, since the surrounding
crystal grains thereof are capable of being deformed into an
arbitrary shape and maintaining deformation of the foil overall,
the result is improvement of plastic deformability.
Moreover, determination of the number of crystal grains in the
direction of sheet thickness depending on sheet thickness makes it
possible to more reliably ensure plastic deformability. Therefore,
this determination is the preferable manner. Since deformation
resistance increases as sheet thickness becomes thicker, the number
of crystal grains should be increased as sheet thickness becomes
thicker. More specifically, in the case of sheet thickness of 15
.mu.m or more, the number of crystal grains in the direction of
sheet thickness is preferably 5 or more, and in the case of sheet
thickness of 40 .mu.m or more, the number of crystal grains is
preferably 10 or more. This makes it possible to further improve
plastic deformability. Furthermore, the effect of sheet thickness
on the number of crystal grains in the direction of sheet thickness
is of a negligible degree in the case of austenitic stainless steel
foil having a sheet thickness of less than 15 .mu.m.
There are no particular limitations on the upper limit of the
number of crystal grains. This is because the number of crystal
grains in the direction of sheet thickness varies according to the
sheet thickness of the austenitic stainless steel foil. There are
no particular limitations on crystal grain size (crystal grain size
as determined in compliance with JIS G 0051 (to be referred to as
"crystal grain size" in the present description unless specifically
indicated otherwise)) provided the number of crystal grains is
three or more. This is because the aforementioned multiple slip is
determined not by the size of crystal grains, but by the number of
crystal grains in the direction of sheet thickness.
[Surface Layer Nitrogen Concentration]
As was previously described, in the case of nitriding the surface
layer of stainless steel foil, specifically when reducing the
thickness of the stainless steel foil, various problems appear due
to hardening of the surface layer caused by nitriding. Thus, the
surface layer of stainless steel foil is preferably not nitrided.
The feature wherein "a surface layer is not nitrided" means that
the nitrogen concentration of the surface layer is 1.0% or less by
mass. Here, the surface layer refers to the thickness at which
nitrogen concentration is half the peak value as determined by
measuring according to Auger electron spectroscopy, and nitrogen
concentration refers to the average concentration thereof in the
surface layer.
Although repeating the previous explanation, in the case the
surface layer of stainless steel foil has been nitrided, since the
surface layer becomes hard due to this nitriding and the location
of that nitriding serves as an origin of tearing during press
forming, press formability ends up decreasing. In the case of the
thin stainless steel foil according to the present invention having
a sheet thickness of 60 .mu.m or less, the effects of the surface
become relatively large, and this problem becomes apparent. Since
making nitrogen concentration to be within the aforementioned range
enables deformation without causing tearing (cracking) of the
surface layer, favorable press formability is obtained.
Consequently, it is preferable to make the nitrogen concentration
of the surface layer to be 1.0% or less by mass as previously
described without increasing the nitrogen concentration in the
surface layer of the stainless steel foil. It is not particularly
necessary to define a lower limit for nitrogen concentration of the
surface layer. The lower limit is equal to the nitrogen content
evaluated for the entire stainless steel foil. Namely, in the case
of steel of a type that does not contain nitrogen such as ordinary
SUS304, the content of nitrogen present in the form of an
unavoidable impurity is the lower limit of nitrogen
concentration.
The nitrogen concentration of the surface layer of stainless steel
foil can be controlled to 1% or less by mass by making the nitrogen
concentration in the annealing atmosphere to be 0.1% or less by
mass.
[Lamination]
The austenitic stainless steel foil according to the present
invention may be laminated austenitic stainless steel foil obtained
by laminating a resin film on the surface thereof in the same
manner as ordinary laminated stainless steel foil. Lamination of a
resin film makes it possible to improve corrosion resistance in
electrolyte and further enhance applicability to a battery case
such as that of a lithium ion battery.
The resin film may be laminated on both sides of the stainless
steel foil or on one side only.
The required level of performance in terms of peeling strength
between the stainless steel foil and resin is obtained by providing
a chromate-treated layer of a suitable thickness on the surface of
the stainless steel foil. For example, Patent Document 5 discloses
a technology for providing a chromate-treated layer having a
thickness of 2 nm to 200 nm on at least one side of stainless steel
foil and laminating a polyolefin-based resin containing a polar
functional group on the surface thereof.
In addition, blushing of the resin following press forming can be
prevented by optimizing the resin design. More specifically, the
resin should be made to be amorphous after heat lamination, and the
cooling rate during heat lamination should be increased in order to
accomplish this. For example, the cooling rate over the range of
120.degree. C. to 80.degree. C. may be 20.degree. C./s.
(2. Method for Producing Austenitic Stainless Steel Foil)
The following provides an explanation of a method for production of
the austenitic stainless steel foil according to the present
invention.
The production process of the austenitic stainless steel foil
according to the present invention is roughly the same as the
production process of ordinary stainless steel foil. Namely,
stainless steel strips are rolled into foil followed by cleaning
the surface, carrying out final annealing and carrying out temper
rolling (using a tension leveler) as necessary to produce stainless
steel foil. Furthermore, the foil rolling step may be divided into
multiple stages corresponding to the sheet thickness of the
stainless steel strips to be subjected to foil rolling (multistage
rolling), and intermediate annealing may be carried out between
each of the foil rolling stages. However, in order to obtain the
austenitic stainless steel foil according to the present invention,
it is important to control the rolling reduction rate during final
foil rolling and the temperature during final annealing as
previously described.
[Rolling Reduction Rate]
Dislocations serving as nucleation sites of recrystallization can
be introduced into stainless steel by carrying out high-pressure
rolling during foil rolling. The number of dislocations introduced
increases as the rolling reduction rate becomes higher. Dislocation
density is controlled by the combination of rolling reduction rate
and annealing treatment after having carried out rolling. Thus, in
the case of having carried out rolling two times or more, the final
round of foil rolling, namely the foil rolling carried out
immediately before final annealing, is preferably carried out under
high-pressure.
More specifically, the rolling reduction rate during foil rolling
prior to final annealing is 30% or more. From the viewpoint of
ensuring dislocation density, rolling reduction rate is preferably
40% or more and more preferably 45% or more.
Furthermore, rolling reduction rate is defined by the equation
indicated below. Rolling reduction rate=(sheet thickness before
rolling-sheet thickness after rolling)/(sheet thickness before
rolling)
Since the objectives of foil rolling include introducing
dislocations in addition to obviously reducing sheet thickness,
there are no particular limitations on the upper limit of rolling
reduction rate. However, since a rolling reduction rate of 100% is
theoretically not possible, the realistic upper limit of rolling
reduction rate is about 95%.
Although depending on the final sheet thickness of the austenitic
stainless steel foil, the lower limit of rolling reduction rate is
preferably 40% or more if possible and even more preferably 45% or
more.
In the case of dividing foil rolling into multiple stages, it is
preferable to control the structure of the material during
intermediate foil rolling and subsequent intermediate annealing. In
this case as well, foil rolling is carried out in the same manner
as final foil rolling. Namely, the rolling reduction rate in each
stage of foil rolling is 30% or more. However, since foil rolling
immediately before final annealing is the most important as
previously described, the rolling reduction rate during final foil
rolling is preferably set higher than the rolling reduction rate
during other stages of foil rolling.
[Annealing Temperature]
Annealing carried out after foil rolling (final annealing) fulfills
the important roles of reducing dislocation density and promoting
recrystallization. As was previously described, an object of the
austenitic stainless steel foil according to the present invention
is to reduce dislocation density and promote recrystallization to
proceed while preventing from grain growth to prevent from
accumulation in specific orientations.
Annealing temperature in the case of the austenitic stainless steel
foil according to the present invention is 950.degree. C. to
1050.degree. C. When the annealing temperature is lower than
950.degree. C., it is difficult to reduce dislocation density, and
recrystallization rate ends up being low. On the other hand, if the
annealing temperature exceeds 1050.degree. C., in addition to
crystals becoming excessively large, orientation proceeds in any of
the aforementioned three orientations, and favorable formability
cannot be obtained. The lower limit of the annealing temperature is
a temperature slightly higher than 950.degree. C., is preferably
960.degree. C. and is more preferably 970.degree. C.
From the viewpoint of preventing from accumulation in specific
crystal orientations, the upper limit of the annealing temperature
is slightly lower than 1050.degree. C., preferably 1040.degree. C.
and more preferably 1030.degree. C.
[Annealing Retention Time]
The amount of time to retain the stainless steel foil at the
aforementioned annealing temperature is 3 seconds to 30 seconds. If
the retention time is less than 3 seconds, heat treatment is
inadequate, recrystallization does not proceed sufficiently, and
the recrystallization rate defined in the present invention cannot
be obtained. On the other hand, if the retention time exceeds 30
seconds, in addition to the recrystallized grains becoming
excessively large, orientation proceeds in any of the
aforementioned three orientations, and favorable formability cannot
be obtained.
[Annealing Atmosphere]
The annealing atmosphere consists of a hydrogen, argon or other
rare gas atmosphere so as to prevent nitriding of the surface of
the stainless steel foil. Furthermore, although it is desirable
that the annealing atmosphere be completely free of nitrogen,
nitrogen for which entry from the air is unavoidable is permitted
to a certain degree. The nitrogen concentration of the annealing
atmosphere should be 0.1% or less by mass in order to make the
nitrogen concentration of the surface layer to be 1.0% or less by
mass.
[Intermediate Annealing]
In the case of employing multiple foil rolling stages, the
temperature during intermediate annealing is preferably 950.degree.
C. to 1050.degree. C. although there are no particular limitations
on the conditions thereof. Since it is desirable that crystal grain
boundaries serve as nuclei of recrystallization and a large number
of recrystallized grains are introduced before foil rolling, it is
preferable to prevent the recrystallized crystal grains from
coarsening by making the intermediate annealing temperature to be
within the aforementioned temperature range.
Examples
Austenitic stainless steel foils having the thicknesses indicated
in Table 1 were produced as examples of the austenitic stainless
steel foil according to the present invention by rolling
commercially available SUS304 steel with a foil rolling mill under
the rolling conditions indicated in Table 1.
Here, rolling reduction rate indicates the rolling reduction rate
in the cold foil rolling step immediately before final annealing,
finish annealing temperature indicates the temperature in the final
annealing step carried out after finishing the rolling process, and
retention time indicates the amount of time the stainless steel
foil is retained at the finish annealing temperature.
The annealing atmosphere consisted of a mixed gas comprising
nitrogen at 0.1% by volume and hydrogen at 99.9% by volume.
Recrystallization rate was obtained by mirror-polishing a
cross-section in the direction of rolling which was to be used as
the observed surface, etching and then observing the cross-section,
and determining the area of crystal grains that recrystallized over
a range defined by total sheet thickness.times.500 .mu.m wide and
calculating the result of the equation: (area of recrystallized
crystals)/(observed area).
The texture of the stainless steel foil following finish annealing
was measured by EBSD after polishing the surface of the foil by
chemical etching or cross-section polishing (CP). The degrees of
accumulation (area %) of crystal orientations in which the
differences in orientation from the copper orientation
{112}<111>, brass orientation {110}<112> and Goss
orientation {110}<001> were within 10.degree. were measured
in a measuring field measuring 100 .mu.m on a side.
Nitrogen concentration of the surface layer was measured by Auger
electron spectroscopy (AES). Measurements were made to a depth of
30 nm from the surface of the stainless steel foil and the average
nitrogen concentration to the depth at which oxygen concentration
is equal to half of the peak value was taken to be the nitrogen
concentration of the surface layer.
The number of crystal grains in the direction of sheet thickness
was determined by cutting out a test piece in the direction of
sheet thickness, carrying out etching after polishing the
cross-section, and observing with an optical microscope, followed
by measuring crystal grain size in compliance with JIS G 0551,
calculating the average crystal grain size, and taking the quotient
obtained by dividing sheet thickness by average crystal grain size
to be the number of crystal grains in the direction of sheet
thickness.
In addition, a chromate-treated layer having a thickness of 10 nm
was provided on one side of the stainless steel foils which were
subjected to finish annealing (final annealing), and a
polypropylene film was laminated thereon, and then a polyester film
or nylon film was laminated on the other side of the stainless
steel foils to prepare samples of which sizes were roughly 100 mm
square. Press forming was carried out in the center of these
samples with a punch of 40 mm high.times.30 mm wide under
conditions of a clearance of 0.3 mm followed by evaluating the
maximum depth at which there was no formation of wrinkles or
cracks. Since maximum forming depth increases as sheet thickness
becomes larger, a forming depth of 4.0 mm or more was evaluated as
being favorable in the case of a sheet thickness of 30 .mu.m or
less, while a forming depth of 4.5 mm or more was evaluated as
being favorable in the case of a sheet thickness of 30 .mu.m or
more. The evaluation results are shown in Table 1.
TABLE-US-00001 TABLE 1 No. of Crystal Grains in Roll- Finish Degree
of Accumulation Direction Surface ing annealing (A1) (A2) (A3) of
Layer Reduc- Reten- Recrystal- Copper Brass Goss A1 + Sheet
Nitrogen Sheet tion tion lization orienta- orienta- orienta- A2 +
Thick- Concentra- Molded New Test thick- Rate Temp. Time Rate tion
tion tion A3 ness tion Depth No. ness (%) (.degree. C.) (sec) (%)
(area %) (area %) (area %) (area %) (no.) (wt %) (mm) Example 1 25
60 950 3 90 1.0 1.5 1.9 4.4 10.0 0.8 5.0 Example 2 25 60 1000 5 100
2.3 2.1 1.1 5.5 7.3 0.6 4.7 Example 3 25 60 1050 5 100 4.1 5.0 4.9
14.0 4.2 0.7 4.3 Example 4 25 60 1050 30 100 4.9 7.3 7.8 20.0 3.0
0.8 4.2 Example 5 5 50 1000 3 100 1.0 2.0 7.0 10.0 3.0 0.5 4.2
Example 6 10 50 1000 5 100 3.0 2.7 4.1 9.8 4.0 0.7 4.3 Example 7 15
50 970 10 100 2.0 2.4 3.6 8.0 5.0 0.6 4.3 Example 8 20 64 1000 15
100 5.4 6.3 5.7 17.4 6.4 0.7 4.2 Example 9 30 70 1030 10 100 5.4
6.2 7.9 19.5 7.2 0.5 4.5 Example 10 40 50 1000 5 100 5.0 3.7 8.6
17.3 11.0 0.8 4.9 Example 11 60 50 1050 10 100 6.4 5.9 7.5 19.8
10.0 0.5 5.0 Comp. Ex. 1 25 60 900 5 50 1.1 1.2 2.5 4.8 15.0 0.5
3.6 Comp. Ex. 2 25 60 1100 5 100 5.5 7.8 9.2 22.5 1.2 0.6 3.9 Comp.
Ex. 3 15 20 1100 5 100 6.5 5.0 9.2 20.7 0.8 0.6 3.0 Comp. Ex. 4 30
70 600 5 0 1.5 1.4 2.0 4.9 9.6 0.6 4.1 Comp. Ex. 5 40 60 1100 10
100 6.8 6.9 8.5 22.2 1.8 0.4 4.3 Comp. Ex. 6 60 20 800 10 20 3.5
3.8 6.0 13.3 8.0 0.5 4.3
As shown in Table 1, in the examples of the austenitic stainless
steel foil according to the present invention, the proportions of
the area occupied by crystal grains accumulated in each orientation
were low, and as a result thereof, forming depth was 4.0 mm or more
in the case of a sheet thickness of less than 30 .mu.m, while
forming depth was 4.5 mm or more in the case of a sheet thickness
of 30 .mu.m or more.
In Comparative Example 1 having a sheet thickness of less than 30
.mu.m, recrystallization did not proceed adequate and
recrystallization rate was low due to the low finish annealing
temperature. As a result, forming depth was less than 4.0 mm.
On the other hand, in Comparative Examples 2 and 3 having a sheet
thickness of less than 30 .mu.m, recrystallization proceeded
adequately due to the high finish annealing temperature and the
orientations of the recrystallized crystal grains accumulated in
each orientation while undergoing further crystal growth. As a
result, deformation anisotropy occurred with respect to stretch
forming and forming depth was less than 4.0 mm.
In Comparative Examples 4 and 6 having a sheet thickness of 30
.mu.m or more, recrystallization did not proceed adequately and
recrystallization rate was low due to the low finish annealing
temperature. As a result, forming depth was less than 4.5 mm.
In addition, in Comparative Example 5 having a sheet thickness of
30 .mu.m or more, recrystallization proceeded adequately due to the
high finish annealing temperature and the orientations of the
recrystallized crystal grains accumulated in each orientation while
undergoing further crystal growth. As a result, deformation
anisotropy occurred with respect to stretch forming and forming
depth was less than 4.5 mm.
According to the above results, by comparing Example 4 with
Comparative Example 2 having the same sheet thickness, a difference
of 0.3 mm or more was able to be confirmed with respect to forming
depth. This difference is extremely significant as indicated below.
Namely, in the case of applying stainless steel foil to a battery
case installed in a compact and lightweight electronic device such
as a smartphone, the battery case is required to have a thickness
of about several mm. Under such circumstances, if forming depth is
large at 0.3 mm or more, this is equivalent to 10% or more of the
thickness of the battery case, which greatly contributes to
increased battery capacity. Thus, the effects of the present
invention are extremely great.
INDUSTRIAL APPLICABILITY
The austenitic stainless steel foil according to the present
invention can be used in a battery case and the like of a lithium
ion battery, for example, for use in compact electronic
devices.
* * * * *